Scheme 4.

Synthesis of substrates 33–35, 37. Reagents and conditions: a) lit. [33] L-proline (0.25 equiv), 2-nitrosotoluene, CHCl₃, −18 °C; b) RMgCl, CeCl₃·2LiCl, THF, −78 °C to rt, overnight; c) acetone, PTSA, 3 h, rt; d) OsO₄ (0.01 equiv), NMO (2 equiv), pyridine, 5 d, rt; e) NaIO₄, MeOH/H₂O, 3 h, rt; f) NaH, (EtO)₂POCH₂CO₂Et, THF, −20 °C to rt, 15 min; g) DIBAL-H, CH₂Cl₂, −30 °C to −10 °C, 45 min; h) Ac₂O, pyridine, CH₂Cl₂, 3 h, rt; i) AcOH/H₂O, 3 h, 60 °C. PTSA = p-toluenesulfonic acid, NMO = N-methylmorpholine N-oxide, DIBAL = diisobutylaluminium hydride.